Signal Theory of Intelligence

7 The development of the segmented nervous system in vertebrates (summary)

The differentiation of sensory modalities between segments described below follows a theory on the evolutionary specialisation of segmented nervous systems developed by Malczan (2019). This theory was presented in detail in *Brain Theory of Vertebrates* (ISBN 978-3-00-068559-0) and in the online version (Malczan 2019).

7.1 The ladder-like nervous system of early segmented animals

The ladder-like nervous system of early segmented animals was initially structured identically in all segments. Each segment possessed the same set of sensory receptors, so that all modalities – smell, touch, temperature, chemical stimuli and others – were equally represented in every segment. The neural networks of the segments were therefore initially functionally identical.

However, as the environment became more complex and the importance of sensory specialisation grew, a process based on neural competition began. Signals from similar receptors inhibited one another via the interneurons at the segment boundaries. Stronger signals suppressed weaker ones. This competition led to a gradual functional differentiation of the segments.

In the uppermost segment – the later cortex – neural competition was most pronounced. Here, the olfactory receptors prevailed: their signals were the strongest and inhibited the olfactory signals of the other segments. In the course of evolution, the olfactory receptors in the lower segments regressed until, eventually, only the first segment possessed olfactory receptors.

The same competitive process was repeated for other sensory modalities. Each modality eventually found a segment in which its receptors and neural networks prevailed. At the end of this evolutionary selection process, each sensory modality was present in only a single segment.

Motor modalities, on the other hand, were retained in all segments, as they were essential for segmental locomotion and body coordination. Similarly, pain receptors and receptors of the autonomic nervous system remained in all segments, as they fulfilled fundamental protective and survival functions.

The separation of sensory modalities led to further differentiation in the uppermost segment: the signal pathways diverged, and the cortex broke down into modal lobes. The neural networks specialised in one modality each and developed their own patterns, weightings and processing strategies. This gave rise to an early form of the brain’s functional organisation.

Another decisive step in evolution concerned the competition between the two sides of the body. This process inevitably led to the emergence of the cerebellum – a topic we will discuss in detail in the following section.

7.2 From the local to the global nervous system: the evolution of global intelligence

The neural networks of early segmented animals described so far operated locally. The range of the axons and dendrites of the interneurons was severely limited, so that each segment could only process information within a narrow spatial range. However, local networks can only produce local intelligence. They recognise patterns occurring in their immediate surroundings and react to them – yet they have no means of integrating information across the entire body.

With the increasing complexity of the environment and the growing importance of coordinated behaviour, evolution faced a new challenge: the creation of globally functioning neural networks. An organism capable of consolidating information from all segments and deriving overarching decisions from it would have a decisive survival advantage.

The solution lay in the development of a globally functioning neural structure present in all vertebrates: the cerebellum. The cerebellum is the first structure to simultaneously receive, process and redistribute signals from all segments. It thus forms the neural basis for the global intelligence of vertebrates.

Whilst the ladder-like nervous system recognised local patterns and controlled local reactions, the cerebellum created an overarching network that coordinated the entire organism. It integrated sensory, motor and internal signals into a global state and influenced the activity of all segments simultaneously.

In this sense, vertebrates could also be described as ‘cerebellate animals’: their capacity for global intelligence, complex coordination and, later, conscious perception is based on the central role of the cerebellum as a global integrator.